milli-charged dark matter

As my loyal reader knows, I love the Brown-Bag talks at the Center for Cosmology and Particle Physics. Today was a great example! Hongwan Liu (NYU) talking about milli-charged dark matter. Putting a charge in the dark sector is a little risky, because the whole point of dark matter is that it is invisible, electromagnetically! But it turns out that if you include enough particle complexity in the dark sector, you can milli-charge the dark matter and move thermal energy from the light sector into the dark sector and vice versa.

Liu was motivated by some issues with 21-cm intensity mapping, but he has some very general ideas and results in his work. I was impressed by the point that his work involves the heat capacity of the dark sector. That's an observable, in principle! And it depends on the particle mass, because a dark sector with smaller particle mass has more particles and therefore more degrees of freedom and more heat capacity! It's interesting to think about the possible consequences of this. Can we rule out very small masses somehow?